Nonaqueous electrolyte secondary battery separator

Abstract

The present invention provides a nonaqueous electrolyte secondary battery separator which is excellent in cycle characteristic. The nonaqueous electrolyte secondary battery separator includes a polyolefin porous film in which a difference between (i) a porosity calculated from a continuous image which is formed so as to extend from a surface to an inside of the polyolefin porous film in a thickness direction and in which a void part and a resin part are shown at respective two gray levels and (ii) a porosity calculated from a thickness, a weight per unit area, and a real density of the polyolefin porous film is 4% to 20%.

The invention claimed is: 
     
       1. A nonaqueous electrolyte secondary battery member comprising:
 a positive electrode; 
 a nonaqueous electrolyte secondary battery separator; and 
 a negative electrode, 
 the positive electrode, the nonaqueous electrolyte secondary battery separator, and the negative electrode being disposed in this order, 
 the nonaqueous electrolyte secondary battery separator including a polyolefin porous film, wherein: 
 the porous film has a real porosity calculated by the following Formula (1) using a thickness (μm), a weight per unit area (g/m 2 ), and a real density (g/m 3 ) of the polyolefin porous film:
   (Real porosity)=[1−(weight per unit area)/{(film thickness)×10 −6 ×1[m 2 ]×(real density)}]×100  (1)
 
 
 the porous film has a continuous image obtained from an FIB-SEM measurement with a magnification of 6500 times and image analysis, the continuous image having a size of 256 pix×256 pix along a surface of the nonaqueous electrolyte secondary battery separator where 1 pix is 19.2 nm, a thickness of the continuous image being equal to a thickness of the nonaqueous electrolyte secondary battery separator, wherein the continuous image extends from the surface to an inside of the nonaqueous electrolyte secondary battery separator in a thickness direction, and, in the continuous image, a void part and a resin part of the polyolefin porous film are shown at two gray levels, wherein the continuous image is segmented into a plurality of images, 
 the porous film has an average porosity calculated from the plurality of images; and 
 wherein a difference between the real porosity and the average porosity is 4% to 20%, 
 wherein the porous film has a weight per unit area of 4 g/m 2  to 20 g/m 2 . 
 
     
     
       2. The nonaqueous electrolyte secondary battery separator member as set forth in  claim 1 , wherein a compressive elastic modulus of said nonaqueous electrolyte secondary battery separator is not less than 1600 kPa. 
     
     
       3. A nonaqueous electrolyte secondary battery member comprising:
 a positive electrode; 
 a nonaqueous electrolyte secondary battery laminated separator; and 
 a negative electrode, 
 the positive electrode, the nonaqueous electrolyte secondary battery laminated separator, and the negative electrode being disposed in this order, 
 the nonaqueous electrolyte secondary battery laminated separator comprising: 
 a nonaqueous electrolyte secondary battery separator and an insulating porous layer, 
 the nonaqueous electrolyte secondary battery separator including a polyolefin porous film, wherein: 
 the porous film has a real porosity calculated by the following Formula (1) using a thickness (μm), a weight per unit area (g/m 2 ), and a real density (g/m 3 ) of the polyolefin porous film:
   (Real porosity)=[1−(weight per unit area)/{(film thickness)×10 −6 ×1[m 2 ]×(real density)}]×100  (1),
 
 
 the porous film has a continuous image obtained from an FIB-SEM measurement with a magnification of 6500 times and image analysis, the continuous image having a size of 256 pix×256 pix along a surface of the nonaqueous electrolyte secondary battery separator where 1 pix is 19.2 nm, a thickness of the continuous image being equal to a thickness of the nonaqueous electrolyte secondary battery separator, wherein the continuous image extends from the surface to an inside of the nonaqueous electrolyte secondary battery separator in a thickness direction, and, in the continuous image, a void part and a resin part of the polyolefin porous film are shown at two gray levels, wherein the continuous image is segmented into a plurality of images, 
 the porous film has an average porosity calculated from the plurality of images; and 
 wherein a difference between the real porosity and the average porosity is 4% to 20%, 
 wherein the porous film has a weight per unit area of 4 g/m 2  to 20 g/m 2 . 
 
     
     
       4. A nonaqueous electrolyte secondary battery member as set forth in  claim 3 , wherein:
 the insulating porous layer contains a polyamide-based resin.